Storr says he expects the reactor to be operating at full power by the end of the month.

OPAL replaced Australia's first reactor, the 40-year-old High Flux Australian Reactor.

It was meant to produce four times the amount of radioisotopes for nuclear medicine than its predecessor and expand the nation's capacity for nuclear medicine research.

But the OPAL reactor was shut down three months after it opened when staff discovered during routine maintenance that some fuel plates had become dislodged and were projecting above, but still attached to the fuel assembly.

Worst case

Storr says in the worst case one of the fuel plates - which are about 2.5 millimetres thick, about 8 millimetres wide and 800 millimetres long - was above the fuel assembly by about 400 millimetres.

Unlike similar reactors around the world, the OPAL reactor design didn't have a secondary mechanism in place to stop the fuel plates moving.

"Not only did we miss it in our review, the designer missed it, ARPANSA missed it and internationally people [who were also tendering for the job] missed it," Storr says.

But he says the movement of the fuel plates cannot just be blamed on the design fault.

ANSTO investigators have concluded the plate movement was caused by three factors.

These are the original design fault, vibrations caused by the rapid flow of cooling water up through the plates and a fault in the manufacturing process for the fuel plates used by Argentinean company CNEA.

Storr says in the OPAL reactor core there are 16 fuel assemblies that each hold 21 fuel plates made up of aluminium and small amounts of uranium.

The two outside fuel plates are screwed in while the 19 internal plates are slid into grooves and 'swaged' or crimped in place.

Readjusted

Storr says the tool used in Argentina to swage the fuel plates had been readjusted and was slightly out.

And tests at the manufacturing site had also shown the vertical strength in the swaging of the fuel plates was "less than expected", he says.

Storr says these factors meant because the plate was not held in place as well as it should have been, once the swaging bond was broken the vibrations caused the plates to move upwards.

To fix the problem, which has been estimated to have cost about A$100,000 a week in lost revenue, ANSTO is now sourcing its fuel plates from a French-based manufacturer.

The design of the fuel assembly has also been changed to include a stopper to prevent further fuel plate movement.

Storr says the new design was tested by leaving all but seven fuel plates completely "unswaged" for 33 days, which is the equivalent of one full operating program.

At the end there was only very slight damage to two fuel plates, he says.

Critics

Under the return-to-service approval ARPANSA chief Dr John Loy has required ANSTO to: develop a program to more fully understand the vibrational and other forces acting on the fuel plates; review the design of the modified fuel assemblies within two years; and regularly test the longitudinal strength of the fuel plates.

Nuclear campaigner for Friends of the Earth, Dr Jim Green, says many critics of the Lucas Heights reactor would have preferred it was never turned back on.

Green, who completed his doctorate on medical isotope supply options, says Australia has no need to manufacture its own medical isotopes as there is surplus supply worldwide.

He says research and development funds should instead be directed towards developing a cyclotron facility that can also produce the necessary isotopes without the safety risks and nuclear waste issues.